i965: Set the type of the null register to fix gen6 FS comparisons.

i965: Set the type of the null register to fix gen6 FS comparisons.

We often use reg_null as the destination when setting up the flag
regs.  However, on gen6 there aren't general implicit conversions to
destination types from src types, so the comparison to produce the
flag regs would be done on the integer result interpreted as a float.
Hilarity ensued.

Fixes 20 piglit cases.